In recent years, 100 Gigabit-per-second (Gbps) long-distance optical transmission has been implemented by dual-polarization quadrature phase-shift keying (DP-QPSK) using a digital coherent technology. To further improve transmission capacity, narrower spectrum bandwidth and higher-level modulation schemes are desired.
In high-speed optical transmission over 10 Gbps, Mach-Zehnder (MZ) modulators are typically used as optical modulators. To generate high-quality optical signals, the operating point of an optical modulator is maintained in the appropriate point with respect to the input drive signals. In phase modulation schemes, a bias voltage applied to the optical modulator is controlled such that the center of the oscillation of an electrical data signal comes to be consistent with the minimum point of the driving voltage to light intensity characteristic curve of the MZ modulator. In order to control the bias voltage to the appropriate value, a low frequency signal is superimposed on the bias voltage, and feedback control is performed on the bias voltage so as to minimize a low frequency (f0) component contained in the output light.
FIG. 1 illustrates eye diagrams of driving signals. Diagram (A) is a drive waveform for QPSK modulation. Diagram (B) is a narrow-band drive waveform for Nyquist-QPSK modulation. Diagram (c) is a multilevel drive waveform for 16 quadrature amplitude modulation (16-QAM). The voltage that causes a change in the light output of a MZ modulator from the maximum light intensity to the minimum light intensity is generally called a half-wave voltage Vπ. By using a drive signal with amplitude 2×Vπ, the maximum output level of a light signal can be obtained. Accordingly, for QPSK modulation, a drive signal having amplitude 2×Vπ is typically used as in the diagram (A).
In contrast, in narrowband transmission in diagram (B) or higher-level modulation schemes in diagram (C), the peak-to-peak driving amplitude becomes greater than the average amplitude of the drive signal. In this case, the average amplitude of the drive signal is set to a value less than 2×Vπ. However, when reducing the amplitude of the drive signal, there is a certain driving amplitude existing that makes the conventional bias control scheme unable to perform bias control.
FIG. 2A and FIG. 2B are diagrams to explain the problem arising when performing bias control using a drive signal with reduced amplitude in Nyquist-QPSK modulation. FIG. 2A illustrates a voltage to light intensity characteristic observed when using a drive signal with average amplitude of 1×Vπ on which a low frequency signal is superimposed. In general, the light output characteristic drifts with respect to the drive signal due to change in temperature or change with time. In the example of FIG. 2A, the operating point deviates or drifts from the optimum bias point (at which the center of the oscillation of the drive signal is coincident with the minimum point of the light intensity characteristic). In this state, when the bias voltage swings to the higher side due to the superimposed low frequency signal f0, the output level of light on the higher-voltage side at the operating point increases (from the trough toward the peak), but the output level of light on the lower-voltage side at the operating point decreases (toward the trough). These changes in the light output level are cancelled out each other. As a result, the low frequency f0 component cannot be detected even if the bias point drifts from the optimum point.
FIG. 2B is a chart illustrating magnitude of the synchronously detected low frequency f0 component as a function of bias voltage for various parameters of the amplitude of the drive signal. From the sinusoidal waveforms “a” through “f”, as the amplitude of the modulator drive signal decreases from 1×2Vπ to 0.8×2Vπ, 0.2×2Vπ, the magnitude of the detected low frequency f0 component becomes smaller. The sensitivity becomes zero at 50% amplitude (i.e., 1×Vπ). When the amplitude becomes less than 50%, the sign or the polarity is inverted and the detection sensitivity gradually increases. When the detected low frequency component is in-phase with respect to the superimposed low frequency signal, the sign is positive. When the detected low frequency component is 180-degree out-of-phase with respect to the superimposed low frequency, the sign is negative.
FIG. 3 illustrates magnitude of the synchronously detected low frequency f0 component as a function of bias voltage for various parameters in 16-QAM modulation. Similarly to the conventional QPSK bias control, a low frequency signal f0 is superimposed on the bias voltage to perform feedback control so as to bring the synchronously detected f0 component closer to zero during 16-QAM modulation. see, for example, Hiroto Kawakami, “Auto bias control technique for optical 16-QAM transmitter with asymmetric bias dithering”, OPTICS EXPRESS, Vol. 19, No. 26, pp. B308-B312, December, 2011. As illustrated in FIG. 3, depending on the amplitude of the drive signal, no low frequency component is detected in spite of the fact that drift of the bias point is observed. In the example of FIG. 3, at the driving amplitude of 0.75×2Vπ, a low frequency component cannot be detected even if the operating point has deviated from the optimum bias point. Besides, depending on the amplitude of the drive signal, the relationship between the in-phase and the 180-degree out-of-phase indicating the direction of the drift of the bias point is reversed.
FIG. 4A and FIG. 4B illustrate a known technique for solving the problem of the presence of a driving amplitude that precludes bias control. In FIG. 4A, an asymmetric combined signal is produced by superimposing a dither signal on the bias voltage, as well as on the modulator drive signal. See, for example, Japanese Patent Laid-open Publication No. 2013-88702. Using the combined signal, the light intensity changes only on the higher voltage side, and a low frequency component can be detected upon occurrence of the drift of the bias point even if the amplitude of the drive signal is 1×Vπ, as illustrated in FIG. 4B. Consequently, as illustrated in FIG. 5, a low frequency component is detected without fail upon occurrence of the drift of the bias point from the optimum point regardless of the designed amplitude of the drive signal.
Another known technique is to superimpose the first pilot signal on the drive signal, while superimposing the second pilot signal on the bias voltage for an optical, to generate a high-quality optical signal even if the amplitude of the drive signal changes. Based upon the first pilot signal component and the second pilot signal component detected from the output light, the bias voltage applied to the optical modulator is controlled. See, for example, PCT Patent Publication WO 2013/114628.
However, with the conventional technique illustrated in FIG. 4A, FIG. 4B and FIG. 5, the target value of the low frequency component used as a criterion for bias control has to be changed according to the driving amplitude. For example, with a driving amplitude of 2×Vπ, the bias voltage is controlled such that the low frequency component contained in the output light becomes zero (see, the sinusoidal curve of dark circle marks in FIG. 5). with a driving amplitude of 1×Vπ (2*Vπ×50%), the bias voltage is controlled such that the low frequency component contained in the output light becomes the maximum (see, the sinusoidal curve of dark triangle marks in FIG. 5). With a driving amplitude of 75%, the bias voltage is controlled such that the low frequency component contained in the output light becomes about 70% of the maximum magnitude (see, the sinusoidal curve of dark square marks in FIG. 5).
With this conventional method, the driving amplitude is always monitored to change the target value of the low frequency component contained in the output light depending on the driving amplitude, and the control operations become complicated and difficult. Taking into account the change with age in the characteristics of the circuit components or temperature change, it is unrealistic to control the bias voltage such that the magnitude of the detected low frequency component becomes a specific value. In 16-QAM, there is a condition where no f0 component is detected in spite of the drift of the bias point depending on the driving amplitude (see, FIG. 3). There is still another problem that the control direction of the bias voltage is switched depending on the driving amplitude.
For these reasons, it is desired to provide a technique for controlling the bias voltage of an optical modulator to the optimum bias point in a stable manner regardless of the employed amplitude of the modulator drive signal.